Field of the Invention
The present invention relates to an electrophotographic image forming apparatus, and more particularly, to a process control thereof.
Description of the Related Art
Image formation using an electrophotographic method includes six processes of charging, exposure, development, transferring, fixing and cleaning. Among them, five processes except the fixing process use a photoreceptor as a direct object to be processed. Thus, in the electrophotographic image forming apparatus, the photoreceptor constitutes a rotating element such as a drum, and a belt, and five kinds of functional units are disposed around the photoreceptor. When the photoreceptor makes one rotation, each portion of the outer circumferential surface sequentially faces these functional units. For these portions, each functional unit specializes in charging, exposure, developing, transferring and cleaning. Thus, while the photoreceptor continues to rotate, each portion of the outer circumferential surface periodically continues to undergo five processes. In order to stably maintain a high image quality in the image forming apparatus, it is important to appropriately control the linkage between the rotation of the photoreceptor and the five processes, depending on switching of operation modes, fluctuation of environmental conditions, and deterioration with time of components.
For example, in an image forming apparatus disclosed in JP 2011-070117 A, an external additive having a lower resistance than the toner is retained in a contact portion between a photoreceptor and a cleaning blade. This external additive escapes charges from the toner scraped off by the blade, suppresses a peeling discharge, and prevents the damage to the photoreceptor caused thereby. However, the peeling discharge is hard to occur in a high humidity environment, whereas the retained external additive is likely to agglomerate into an excessive lump. This lump has a risk of hindering adhesion of toner to the surface of the photoreceptor and disturbing (filming) a horizontal stripe in the toner image. Therefore, when the risk of peeling discharge is low due to a high humidity, the device rotates the photoreceptor in reverse at the time of completion of the job process and levels the external additive to prevent its aggregation.
A copying machine disclosed in JP 2014-021261 A continues to rotate the photoreceptors for other colors even in a monochrome mode to keep job process at high speed and maintain high productivity. In this case, the copying machine periodically discharges toner to the photoreceptors to suppress the frictional force with the cleaning blade. This prevents troubles such as jitter, squealing and curling caused by excessive chatter (stick-slip) of the blade. The copying machine further applies a charging bias to the surface portion of the photoreceptor including the discharged toner after passage of the blade. This charging bias eliminates the charged state, even if the peeling discharge generated between the surface portion and the blade charges its surface portion. As a result, since the carrier contained in the two-component developer does not adhere to the surface portion thereof, the risk of filming or damage to the photoreceptor caused by the carrier is suppressed.
In the process control, the order of startup/stop of each element of the image forming apparatus accompanied by the start/end of the job process is restricted under various conditions. This order itself or the startup/stop control of this order is called a “rising/falling sequence”. Specifically, for example, in the falling sequence, the stopping order of driving, charging, developing and transferring of the photoreceptor is defined as follows. 1. The polarity of the transfer bias is reversed at the time when the termination of the portion containing the toner image among the surface of the photoreceptor passes through the transfer unit. 2. The application of the charging bias is stopped at the time when the termination passes the charging unit. By keeping the charging bias up to this time, the portion before the termination is uniformly charged. 3. The application of the developing bias is stopped at the time when the termination has reliably passed through the developing unit. By keeping the developing bias up to this time, the carrier contained in the two-component developer is prevented from adhering before the termination. However, at this time point, since the uncharged portion just after the termination thereof receives the developing bias, toner (hereinafter, referred to as “fogging toner”) adheres to this portion. 4. The application of the transfer bias, a drive motor of the photoreceptor, and an eraser are stopped at the later of the time when the termination passes through the eraser or the time when the termination passes through the cleaning blade. By keeping the transfer bias at the opposite polarity up to this time, movement of the fogging toner to the transfer unit is prevented, and by continuing to operate the eraser, the portion before the termination is discharged. Furthermore, by continuing to rotate the photoreceptor until this time, the blade is caused to scrape off the fogging toner by this time or after this time while the photoreceptor rotates with inertia.
In recent years, the particle diameter of the toner is reduced in order to further improve the image quality, and the rotational speed of the photoreceptor increases in order to further improve the productivity. Accordingly, in the falling sequence, there is a high risk of occurrence of peeling discharge between the cleaning blade and the photoreceptor at the time of removing the fogging toner. This is due to the following reason. A. With an increase in the speed of the rotation of the photoreceptor, the uncharged portion of the surface of the photoreceptor that receives the developing bias expands and the amount of fogging toner increases. B. With a decrease in the particle diameter of the toner, the surface shape of the lump of the toner scraped off by the blade is miniaturized and electric field concentration is easily generated. C. With an increase in the frictional force received by the photoreceptor from the blade, the amount of charge induced on the surface of the fogging toner peeled off from the surface of the photoreceptor increases.
Meanwhile, in the falling sequence, the eraser and the drive motor of the photoreceptor are stopped, on the later of the time when the termination of the portion of the photoreceptor surface which receives the charging bias passes through the eraser and the time when the termination passes through the cleaning blade. Therefore, generally, the uncharged portion immediately after the termination thereof cannot be subjected to the discharging process by the eraser after passing through the blade. Therefore, when the uncharged portion is charged by the peeling discharge, the carrier adheres to the uncharged portion, while the photoreceptor continues to rotate by inertia, or when the drive motor of the photoreceptor is started up next. These carriers have a high risk of causing image quality deterioration due to filming or a damage to the photoreceptor. However, it is difficult suppress this risk with the above-described falling sequence.